This has always been an interesting subject to me in regards to theories about Berserk. Personally, I really like the subplot about the Beast of Darkness getting into Guts' head and basically telling him to MURDER EVERYTHING. But a thought always crossed my mind.

Is the Beast of Darkness actually there? From what I've seen, only Guts is the one that notices it. His Brand, assuming I am not forgetting a part, never activates when the Beast of Darkness is talking to him. The only solid evidence I've seen where the Beast of Darkness might be an actual demon is when the Berserker Armor changes when he goes nuts. Even then, they already established that the armor was cursed beforehand, maybe even possessed.

So was the thing in the armor taking the form of the Beast of Darkness to mock Guts, or was it the other way around, and that the thing used the armor to manifest?

Basically, did Guts go crazy and imagined it? Or was it a literal demon following him around, like the others?

It doesn't have a tangible existence, because it's a mental manifestation of the trauma Guts endured as a result of the Eclipse, and during the time that he drenched himself in darkness, hunting apostles.

However, to your question, when it first appears, the specters that surround Guts in the forest reflect its form, as they're able to sense the darkness within Guts, which they say is akin to their own.

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they already established that the armor was cursed beforehand, maybe even possessed.

It's a cursed armor, but it's not possessed. In Schierke's words, it has a malevolent od (aura / existence) that pushes its owner to their limits. Guts had something of a preexisting condition, given the Beast of Darkness within him, which took advantage of the armor's nature to take control. But it's still all Guts, not some demon.

Personally, I really like the subplot about the Beast of Darkness getting into Guts' head and basically telling him to MURDER EVERYTHING.

To be precise, what it really wants is for Guts to go after Griffith, forgoing everything else. The problem is that Guts has chosen to stop chasing his revenge in order to protect his lover, Casca. And that just goes directly against the very nature of the Beast of Darkness. And now he's also found new friends, even worse! As a result, the Beast would want nothing more than for them all to die, clearing the way for Guts to completely lose himself, once and for all, in his quest for vengeance.

Is the Beast of Darkness actually there? From what I've seen, only Guts is the one that notices it.

I'm repeating what Walter said a bit, but basically no, the Beast of Darkness isn't really there. In fact, when it talks to Guts, it's done in such a way in Japanese that it can be interpreted as Guts talking to himself (which is basically what's happening). The Beast of Darkness is a personification of Guts' dark side and emotions, meaning that Miura decided to give the abstract "bad feelings" that made Guts into the Black Swordsman a form. He made them into a character, which is pure genius if you ask me. That adds a layer of complexity to Guts' character, as he has to deal with the psychological trauma he endured during the Eclipse and afterwards in addition to everything else. That also makes him that much closer to Casca, who deals with her own trauma in a different way. And it does all that while looking super cool.

The only solid evidence I've seen where the Beast of Darkness might be an actual demon is when the Berserker Armor changes when he goes nuts. Even then, they already established that the armor was cursed beforehand, maybe even possessed. So was the thing in the armor taking the form of the Beast of Darkness to mock Guts, or was it the other way around, and that the thing used the armor to manifest?

The Berserk's armor is cursed because of the way in which it works. Basically, it's a magical item that suppresses people's pain and fear, that de-inhibits them, while emphasizing their fighting instinct. That makes them push themselves beyond the limits of the human body. It manages to do that because its Od (spiritual essence if you will) is like a great flame that engulfs the Od of the one who wears it. The cursed part comes from the fact that the user doesn't care anymore whether or not they're injured. They just keep going and going, fighting mindlessly. And the stronger their will to fight, the stronger the effect is. That means they also don't fight particularly intelligently (e.g. not avoiding damage while they could have), and don't necessarily recognize friend from foe anymore. Hence the name.

But to get back on what we were saying: we know the Beast of Darkness is a representation of the dark side within Guts' mind. And we know that the Berserk's armor works by boosting the wearer's bellicose side and suppressing everything else. Therefore, what better symbol for it to take than the Beast of Darkness? It is what symbolizes Guts' rage and hatred against his sworn enemies. Also, note that this is something the armor does for everybody, as far as we know. It's not specific to Guts. Case in point: the armor's previous user was the Skull Knight, which is why its helmet was shaped like a skull when we first saw it. It adapts to a new user when it is first worn, taking the shape that best mirrors their psyche.

In the very first episode, Guts says to Puck that he doesn't care who will die because of him. He also says something along the lines "everyone else will drag me down" Puck says it's something more complex than sadness fear and hatred. The emotions Puck felt I would say is the Beast of Darkness.

Puck says it's something more complex than sadness fear and hatred. The emotions Puck felt I would say is the Beast of Darkness.

The emotions he felt were of course Guts' own, but indeed those exemplify the aspects that made the Beast.

There's a similar moment in Vol 18, when Guts gets the sudden memory of the Eclipse after SK says it will happen again. Puck senses a sudden rush of emotion in Guts, DH translates it as "ecstasy," I've also seen it translated as "joy," but you get the picture. It's the aspect of Guts that wants him to throw everything away and embrace his rage.